The invention relates to a device to compress combustion air, in particular an electrically operated charge-air compressor.
Increasing the power density of an internal combustion engine by compressing the air required for combusting the fuel by means of an exhaust-gas turbocharger is known. In this case, the exhaust-gas turbocharger features a turbine, which is arranged in the exhaust-gas flow of the combustion engine and which drives a compressor arranged in the charge-air feed of the internal combustion engine.
These types of exhaust-gas turbochargers have the known disadvantage of delayed and inadequate response characteristics at low rpms of the internal combustion engine (“turbocharger gap”).
In order to improve the charge-air feed, especially in the range of lower rpms of the internal combustion engine, supporting the exhaust-gas turbocharger by means of an electric auxiliary drive is known. This can be achieved, for example, via an electric motor integrated into the exhaust-gas turbocharger. The electric motor then drives the shaft of the compressor of the exhaust-gas turbocharger at low rpms of the internal combustion engine. This kind of hybrid drive of the exhaust-gas turbocharger requires both a high rpm loading capacity of the electric motor as well as a high electric power requirement because of the high moment of inertia of the turbine of the exhaust-gas turbocharger that is necessarily heat-resistant and embodied of steel. In addition, the installation volume of such a exhaust-gas turbocharger is increased by the integration of the additional electric motor.
In order to avoid the disadvantages of the this type of hybrid drive, operating a separate, purely electrically operated auxiliary charger (electric auxiliary compressor) in the charge-air feed of an internal combustion engine in series with a conventional, turbine-operated exhaust-gas turbocharger is known from U.S. Pat. No. 6,029,452, for example. This has the great advantage that the electric auxiliary compressor that is used separately from the charge-air feed can be optimized for a short-term use in the lowest rpm range of the internal combustion engine. Typically, the electric auxiliary compressor is connected in series with the exhaust-gas turbocharger so that the achievable charge-air pressure is yielded as a product of the individual pressure values.
The short-term, very high-speed operation of the electric auxiliary compressor produces very high thermal stress to these system components. In addition to passively deriving the excess heat, also cooling the electric auxiliary compressor itself in operation via the inducted charge air is also known. For active air cooling, it is proposed in U.S. Pat. No. 5,904,471 that the inducted air flow first be directed along the motor housing or directly through the motor housing and then be fed to the compression space of the auxiliary compressor. The arrangement of the compressor impeller on the side of the driving motor opposite from the induction opening as well as guiding the airflow through the motor itself results in an increased flow resistance for the passive charge-air compressor. Such a system has the disadvantage of lower efficiency that can be achieved for the charge-air pressure.
In the range of high rpms of the internal combustion engine, which simultaneously leads to a high rpm of the exhaust-gas turbocharger and is therefore also connected with a high throughput of charge air alone due to the conveyance effect of the exhaust-gas turbocharger, a bypass solution is used to feed the charge air directly to the compressor of the exhaust-gas turbocharger while bypassing the auxiliary compressor that is now no longer needed. As a result, it is necessary for the electric auxiliary compressor to have, in its passive operation, the lowest possible flow resistance of the charge air inducted by the exhaust-gas turbocharger.
Integrating a bypass channel in the electric auxiliary compressor is known from U.S. Pat. No. 5,904,471. A flap valve used in U.S. Pat. No. 5,904,471, which diverts the charge-air flow into the bypass channel so that it is not directed via the compression space and the therein arranged compressor impeller of the electrically operated turbo-compressor. This type of flap-controlled bypass solution is not optimal, however, with respect to the flow-related requirements of the auxiliary compressor and is very costly and expensive for charge-air compression in terms of the requirements for assembly, size and cost of the overall system.
In particular, increased flow resistances are produced in the auxiliary compressor with a bypass solution in accordance with U.S. Pat. No. 5,904,471 since, among other things, corners and edges that produce turbulent flow conditions are present in the flow channel due to the embodied flap valve.